Pharyngeal jaws - Fishes

Depression of the mouth floor also creates water flow towards the throat, thereby helping push food items posteriorly. Here the prey encounter a second set of jaws, the pharyngeal apparatus (see Division Teleostei).Pharyngeal jaws evolved from modified gill arches and their associated muscles and ligaments. The lower pharyngeal jaws are derived from the paired fifth ceratobranchial bones, whereas the upper jaws consist of dermal plates attached to the posterior epibranchial and pharyngobranchial bones. Both jaws bear teeth that vary depending on the food type of the fish (see below). Dentition not only varies functionally among species that eat different food types, but may develop differently among individuals of a population as a function of the food types encountered by the growing fish. In the Cuatro Cienegas Cichlid of Mexico,Cichlasoma minckleyi, fish that feed on plants develop small pappiliform pharyngeal dentition, whereas thosethat feed on snails develop robust molar formdentition(Kornfield& Taylor 1983).

In their simplest action, pharyngeal jaws help rake prey into the esophagus. They may additionally reposition prey,immobilize it, or actually crush and disarticulate it. These actions involve at least five different sets of bones and muscles working in concert, including 10 different muscle groups and bones of the skull, hyoid region, lower jaw, pharynx, operculum, and pectoral girdle. The main action is the synchronous occlusion (coming together) of the upper and lower pharyngeal jaws. In cichlids, prey is crushed between the anterior teeth of both pharyngeal jaws, pushed posteriorly by posterior movement of both jaws, and then bitten by the teeth of the posterior region of the jaws (Lauder 1983a, 1983b, 1985).

Pharyngeal pads and their function as jaws influence feeding in another important manner. Gape limitation, the constraint on prey size imposed by mouth size, is in part determined by oral jaw dimensions: a fish can’t eat anything it can’t get into its mouth. But gapelimitations also influenced by pharyngeal gape. If a prey itemis too large to pass through the pharyngeal jaws, it is also unavailable to the predator. Hence many predators can capture but not swallow a prey item because of pharyngeal gape limitation. In small-mouthed species, such as the Bluegill Sunfish, oral and pharyngeal gape differ only by 20–30%. But in piscivores that use oral protrusion for prey capture, such as the Largemouth Bass, oral jaws may be twice the size of the pharyngeal jaws, which means that usable prey size is considerably smaller than that which can be engulfed by the mouth. Posterior to the pharyngeal jaws is the throat, the width of which is determined by spacing between the cleithral bones of the pectoral girdles. Thus a predator can only eat prey that can pass through its oral jaws, pharyngeal jaws, and intercleithral space (Lawrence1957; Wainwright & Richard 1995).

A crucial function of the pharyngeal apparatus in many species is therefore to crush prey to a size small enough to pass through the throat. Here prey morphology comes into play, because prey that is just small enough to fit between the pads may be too hard to crush and is thus unavailable to the predator. This interplay of structure, function, and the constraints created by the pharyngeal apparatus is shown nicely in Caribbean wrasses that feed on hard-bodied prey (Wainwright 1987, 1988a). Wrasses, along with other“pharyngognath” fishes such as parrotfishes and cichlids, have a highly modified pharyngeal apparatus that can crush hard-bodied prey. The size of the muscles that move the pharyngeal jaws differs among three species, the Clown Wrasse (Halichoeres maculipinna), Slippery Dick (H. bivittatus), and Yellow head Wrasse (H. garnoti). In all three species, muscle mass and pharyngeal gape increase with

increasing body size (Fig. 8.6).

Figure 8.6

Crushing ability of the pharyngeal jaws in three related wrasses as a function of body size. Larger wrasses can crush larger snails because of their stronger pharyngeal jaws, and differences among species also influence preferred food types. Clown Wrasses have relatively weak jaws and feed on relatively soft-bodied prey, particularly when the fish are younger. Slippery Dicks and Yellow head Wrasses have strong jaws and feed on shelled prey throughout their lives. After Wainwright 1988a;fish drawing from Gilligan (1989).

At any size, Clown Wrasses have smaller pharyngeal musculature than the other two species. Small Slippery Dicks and Yellow head Wrasses can crush and eat snails that are unavailable to larger clown wrasses. Small Clown Wrasses cannot crush even small snails. These abilities are reflected in the natural feeding preferences of the species. Small clown wrasses feed preferentially on relatively soft-bodied crabs and other invertebrates; they shift to snails only after attaining a body length of 11 cm, when they eat hard-bodied prey that are smaller than those taken by equal-sized fishes of the other two species. Slippery Dicks and Yellow head Wrasses feed extensively on snails beginning at a relatively small fish body length of 7 cm. Pharyngeal crushing strength accounts forinter- and intra specific differences in feeding habits in these fishes; competitive interactions and optimal prey characteristics other than shell strength have little if any influence.

As is so often the case in evolution, an adaptation opens up opportunities that become selection pressures favoring additional innovations. In moray eels, a remarkable modification of the pharyngeal jaws occurs. Morays develop weak suction pressures in the mouth cavity, which limits the rearward pushing of prey, as mentioned earlier. This is additionally complicated by the crevices and other tight places where morays often feed, which constrains jaw movements that would normally aid in swallowing. Morays have “solved” this dilemma by developing raptorial pharyngealaws. The upper, pharyngobranchial arch and the lower, ceratobranchial arch are equipped with sharp, highly recurved teeth not unlike those in the oral jaws. In theresting position, the pharyngeal apparatus sits far back in the throat, behind the skull. But when prey are grasped by the oral jaws, a series of muscles project the pharyngeal jaws far forward into the oral cavity, these jaws then grasp the prey and retract back, pulling the prey toward the esophagus (Mehta & Wainwright 2007).